There is the illegal immigration purity theory, where everyone thinks that securing the border (ever how it's done) and sending all illegals back to their origins (if that can be done, doubtful, IMO) will solve the problem. And there's illegal immigration reality, where Texas has been dealing with the political and economical realities of mass illegal immigration.
Texans can speak to this better than I can, but I suspect that Republicans hold on Texas has been slipping because of the hard stance on illegal immigration. If we lose Texas it is over.
Link to "On Texas and Presidential Politics" at RushLimbaugh.com